Denim fabric is defined as that in which the warp of the fabric, normally of cellulosic fibres, is mainly dyed with indigo blue dye and that with rubbing and use and/or washing with mechanical or chemical means gradually loses its colour. The main property of indigo blue dye and the type of dyeing process is that the dye is deposited on the yarn in superficial concentric rings and that the tincture does not penetrate inside the yarn and fibres.
There are also fabrics on the market with a Denim appearance that are dyed with other types of dye, including sulphurous, pigmented, direct prior cationization of the yarn, etc. But these fabrics are not defined as authentic Denim but are imitations and do not have the properties of Denim dyed with indigo blue dye.
There are various types of Denim fabric or with the appearance of Denim that have fire-retardant properties; the most important of which are listed below:
Denim fabrics made fire-retardant by a process of finishing the fabric by foulard (dipping bath), dyed with indigo, sulphurous, acidic or pigmented dyes, with or without prior cationization, where these fabrics, which could be of various types of composition such as 100% cotton fabric or a mixture of mostly cotton with polyester or cotton with polyamide 6.6, or cotton with Lyocell, etc., are made fire-retardant by a process of finishing by foulard, continuous or by wringing, also having finishes with chemical treatments based on phosphoric acids such as Proban® and Secan®.
Other types of fabrics are those that imitate the appearance of Denim fabric and are based on permanently fire-retardant fibres dyed in bulk, in flock, skein or crossed reel with a colour that imitates indigo blue colour. These fabrics are used as warp yarns and are woven with whitish ecru colour yarns, preferably linked with 3/1 or 2/1 twill weaves to obtain fabrics that visually imitate the appearance of a Denim fabric, but do not wash out and lose colour with washing or rubbing with use (example: Kermel®).
Yarns are also known that are used for producing fabrics such as those previously mentioned, preferably obtained by conventional cotton spinning processes, also using open-end yarns, and in some cases yarns from worsted spinning, being able to use all yarns with an end twisted by a conventional system, and there are also parallel yarns covered by core-twist technology (e.g. polyester, polyamide, polyamide-imide, meta-aramid, para-aramid). These yarns are used for the core in the core-twist spinning process, which consists of covering a yarn of normally continuous parallel and non-textured filaments so that a core or continuous normally synthetic and non-textured internal yarn is covered by a double helicoidal covering of two cut fibre yarns. The texturing and increase in volume and matting of the continuous yarn is obtained by friction or Airtex discs.
With respect to finishes, there are chemical products on the market to obtain a fire-retardant finish on 100% cotton fabric, or a mixture of cotton with synthetic fibres, based on chemical compounds where the active ingredient is mainly phosphoric acid or tris(aziridinyl)phosphine oxide (APO), being normally applied by foulard and wringing, thermofixing for polymerisation, with a subsequent reduction wash and rinsing to remove residual product and finally neutralising the pH of the fabric.
The usual process based on impregnation and crosslinking polymerisation treatment is as follows:                Firstly, the fabric is prepared by drying to remove excess moisture.        Secondly, the cotton fabric is treated with a chemical solution containing phosphoric acid in a process of impregnation by foulard, so that the dry fabric is cured using ammonia gas in the curing process, which causes the small molecules to join to form a polymer, which is cross-linked and bound to the core of each of the fibres.        Next an oxidation process is performed and finally the cloth is washed thoroughly in a 1.2% reducing bath at 60° C., rinsing in water and a subsequent neutralisation, followed by drying at 130° C. for one minute.        Another fire-retardant finishing system is based on crosslinking with hydroxy-functional organophosphorus oligomer.        
The drawbacks or unresolved issues presented by these aforementioned processes are as follows:                This range of fabrics cannot guarantee to be permanently fire-retardant, nor retain the initial level of fire-retardant performance because they lose these properties after washing, which is necessary for keeping the fabric in use.        These types of finish also leave the fabric more rigid, reducing its comfort and the chemical agent used can cause allergies in some people.        The fabrics are obtained from synthetic or artificial fibres such as meta-aramid, para-aramid, polyamide-imide, PBI, modacrylic, chlorofibres, all fabrics that are not comfortable because they do not have natural or cellulose fibres in their composition.        There are fibres that imitate indigo colour, but the colour does not fade on use and washing like authentic Denim, and normally use uniform colours.        There are fabrics composed of fibres that do not have good hygroscopicity, or capillarity, so therefore they do not absorb or regulate moisture, resulting in poor breathability with low Rec and Ret values (according to EN 31092 standard Determination of the physiological/thermal properties and to water vapour).        The yarns used for this fabric range are obtained by conventional cotton spinning processes, using open-end yarns, that are twisted by conventional systems, in some cases applying yarns obtained by worsted spinning and parallel yarns coated by core-twist technology.        Finally, the yarns obtained, as well as the resulting fabrics, are characterised by being rigid and rough to the touch.        
In any case, documents corresponding to patents are known that refer to or are related to indigo dyeing processes. In this sense, document ES2059745 refers to some compositions of dyes for dyeing drill cotton yarn and a process for dyeing and articles dyed using this process, claiming a dyeing process that involves immersion of the yarn in an indigo bath, with a phase of draining and one of oxidation by ventilation of the dyed yarn.
There is also document ES2060047, that concerns a process of dyeing strands using an indigo derivative and the dyed article obtained by this process that comprises repeated immersion in a dye bath, draining of excess liquid and oxidation by exposure to air.
There is another document U.S. Pat. No. 4,131,423 that refers to a process for dyeing cellulosic fibres that enables binding of the tincture by oxidation treatment.
Another document is ES8406600 that refers to a process of neutralisation of cellulosic substrates impregnated with a free or bound alkaline metal hydroxide, performing the neutralisation in situ by contact with a neutralising fluid containing carbon dioxide.
Another document is the patent US20060059635 that refers to a method that enables satisfactory adhesion of the tincture on the surface of fibres by the preparation of a bath including dye or sulphur dye pigment particles and some additives that cause the pigment to be electrically charged. Next, a second additive is applied, in this case on the textile material to create a substrate with opposite polarity to the first polarity created. Then the textile material is immersed in the prepared bath to cause the pigment particles to be attracted by the substrate and retained. Finally, the retained pigment particles are chemically reduced and then the reduced pigment particles are oxidised to form the pigments within the textile material.
Another document that may be mentioned is patent US2008280519 that refers to a process for dyeing cotton fibres mixed with synthetic fibres for the manufacture of coloured “Jeans” by a direct or cationic dye, so that a colour-fast cloth is obtained that will not be washed out or discoloured with use or washing as with indigo. However, after study it was observed that the process comprises a scouring stage where the yarn passes through a water bath at 40-90° C. containing a cationic wetting agent, the dyeing stage contains a buffer solution and water at 60-90° C. with a cationic dye, the concentration of which varies between 0.05 and 500 g/l, then a first wash, a stage of binding the dye to the cloth by a water bath at room temperature and an anionic arylsulphonate binding at a concentration of 80 to 120 g/l and, finally, a second wash.
Another document is Korean patent KR20120076096 A, where a process is described that involves the use of natural indigo in powder for dyeing “Jeans”, which may be of Lycra or Denim, comprising the stages of drying, powdering the indigo, mixing the powder with hydrosulphite, sodium hydroxide and water, treating at 40-60° C. for 20-40 minutes, immersion of the pair of “Jeans” in the dyeing solution, oxidation, washing, neutralisation with acetic acid and washing.
Another document is patent US2005204488 that refers to a process for dyeing natural or cellulosic fibres combined with artificial fibres with indigo dye, where the natural fibre is at least 10%. This document refers to the impregnation of the unwoven fabric in a bath containing leuco-indigo dye.
Finally, there is a document corresponding to a Japanese patent JPH10280286 A, describing dyeing of synthetic yarn with indigo using a conventional process.
In any of these cases, that is in all the processes described in the documents listed above, a complete process of indigo dyeing of synthetic fibres with fire-retardant properties, as described in this patent application, is not performed.